Redox and Standard Electrode Potential

Redox and Standard Electrode Potential

Redox Reactions

  • A redox reaction involves reduction and oxidation, where one species loses electrons (oxidation) while another gains electrons (reduction).
  • Oxidation is loss of electrons, and reduction is gain of electrons. Remember this using the acronym ‘OIL RIG’.
  • Redox half-equations are equations that show either the reduction or oxidation part of a redox reaction.

Oxidation States

  • An element’s oxidation state is a measure of the degree of oxidation or reduction of an atom in a chemical compound.
  • The sum of oxidation states in a neutral molecule always equals zero, while in a charged ion it equals the charge of the ion.

Standard Electrode Potentials

  • The Standard Electrode Potential (SEP) is the voltage of a half-cell compared to a standard hydrogen half-cell, under standard conditions.
  • SEP values allow predictions about the direction of redox reactions. Reactions can only occur if the overall cell potential is positive.
  • Half-cell reactions with more positive SEP are favoured to proceed as reductions, while those with more negative SEP tend to proceed as oxidations.

Predicting Feasibility of Redox Reactions

  • To predict whether a redox reaction is feasible, write out the half-equations for the oxidation and reduction processes and then add the SEP values.
  • If the resultant cell potential is positive, the reaction will occur spontaneously.
  • However, a positive cell potential does not guarantee a fast reaction – it can still be slow if the reaction has a high activation energy.

Using SEP To Predict Reaction Direction

  • When using SEP to predict the direction of a redox reaction, remember that the species with the more positive potential is reduced and the species with the more negative potential is oxidised.
  • The direction of redox reactions can change with different reactant and product concentrations, or under non-standard conditions.

Electrochemical Cells

  • An electrochemical cell consists of two half-cells where redox reactions occur.
  • Each half-cell is made from a metal electrode in a solution of its ions.
  • The metal electrode where oxidation occurs serves as the anode, and the electrode where reduction occurs is the cathode.
  • Connecting the two electrodes allows electron flow through an external circuit, which is an electric current.